In early 1937, Wolff set off to join the International Brigades in Spain, reaching Albacete by March. As a pacifist, a belief common in the 1930s, he originally wished to be a medic.[2] However, after the International Brigades' heavy losses at the Battle of Jarama, he became a soldier instead, joining a machine gun company.[2] "Largely self-educated, ... [he] was an intellectual".[3] He "detested elegant uniforms", customarily wearing "baggy trousers, a stained leather jacket" and, in wet weather, a "woolly poncho".[3]

After a year's fighting in Brunete, Belchite and Teruel, the Brigade lost two senior officers, David Doran and Robert Hale Merriman at the Gandesa battle on the Aragon front. After which, in March 1938, Wolff became the battalion commander.[2] He led the now Lincoln-Washington Battalion during the Battle of the Ebro and left Spain in November 1938 when the International Brigades were demobilized. Ernest Hemingway described him during this period: [he was] "...23 years old, tall as Lincoln, gaunt as Lincoln, and as brave and as good a soldier as any that commanded battalions at Gettysburg. He is alive and unhit by the same hazard that leaves one tall palm tree standing where a hurricane has passed."[4]

In 1940, Wolff volunteered for the British Special Operations Executive, and arranged arms for the European resistance organizations. After the United States' entry into World War II, Wolff volunteered for the infantry in June 1942.

Wolff appeared before the House Un-American Activities Committee to defend VALB (Veterans of the Abraham Lincoln Brigade) from being banned as a Communist front organization. His explanation for his actions owed to his ancestry: "I am Jewish, and knowing that as a Jew we are the first to suffer when fascism does come, I went to Spain to fight against it."[5]

According to historian Peter Carroll:

When Congress passed the McCarran Act in 1950, obliging all designated subversive organizations to register with the federal government and creating heavy penalties for leaders who refused to cooperate, the entire executive committee of the VALB resigned in 1950. In its place, two Lincoln veterans stepped forward: Wolff became the National Commander; Moe Fishman became the Executive Secretary/Treasurer and served the organization in an executive capacity for the rest of his life.[6]

Wolff also battled fiercely for civil rights and against the Vietnam War. He even offered the services of the aging veterans of the Lincoln Brigade to the North Vietnamese leader, Ho Chi Minh, who declined them. Later, Wolff campaigned against apartheid in South Africa, and raised money for ambulances in Sandinista-ruled Nicaragua in the 1980s, personally delivering twenty of them.[2] Wolff completed two autobiographical novels, A Member Of The Working Class about his early life in New York, and Another Hill about his communist and Spanish experiences; he began a third book, The Premature Anti-Fascist, describing his experiences after leaving Spain and during World War II, but could not bring himself to finish it.

“

This extraordinary novel centers on one battalion, the Americans, known as the Lincolns, barely trained men who went into battle armed with 1903 Remington rifles. I have never read more intimate, convincing, and devastating accounts of combat. ~ Martha Gellhorn on Another Hill

Wolff married and had two children. His family resided primarily in Stony Brook, New York. He supported his family as a pharmaceutical salesman, which required that he live in New York City mid-week and with the family on the weekends. Eventually this marriage ended in divorce. While Wolff was in California, he looked up a comrade of his from Spain, a fellow New Yorker. Her sister had died years before, and for reasons of political prejudice and identity, [clarification needed] she took her sister's name. She was an artist who owned a dress shop in Carmel. She sold her dress shop, and they left to see the world. Wolff learned that she had been diagnosed with cancer, and part of their travels included a stop in Aguascalientes, Mexico, where they hoped that the local cures might send the disease into remission. Following their travels, they settled in El Cerrito, California. Wolff and his second wife are both buried in Kensington.